Ceramic coated corrosion-resistant product



United States Patent Office 3,197,291 Patented July 27, 1965 3,197,291CERANHC CQATED CGRROSION-RESISTANT PRODUCT Harold J. Michael, Columbus,Ohio, assignor to North American Aviation, Inc.

No Drawing. Original application Apr. 17, 1961, Ser. No. 103,243.Divided and this appiication 31:53! 19, 1962, Ser. No. 211,105

' 1 Claim. (Cl. 29-195) This invention relates generally tohigh-temperature ceramic protective coatings, and particularly concernsa product having material which readily oxidizes or corrodes whenexposed to oxidizing or corroding environments at elevated temperaturesand having a ceramic coating provided thereon which develops improvedresistance against such oxidation or corrosion especially attemperatures from approximately 1700" F. to approximately 2150 F.

Also, this application is :a divisional application of applicationSerial No. 103,243, filed April 17, 1961.

As used in this application, the term high-temperature ceramicprotective coating refers to a protective coating which is essentiallycomprised of glass frit and refractory constituents and which has anoxidation or corrosion protection service temperature upper limit in therange of from approximately 1700 F. to approximately 2150 F. In thepractice of my invention, such high-temperature ceramic protectivecoatings are applied to the to-be-protected parts in slip form and arefired to maturity in an air atmosphere at a temperature which is atleast as high as approximately 1700 F.

Much difficulty has been experienced in adequately protecting oxidizableor corrosion-susceptible metallic surfaces against deterioration whensuch surfaces are to be subjected to an oxidizing or chemicallycorroding environment at an elevated exposure temperature. Particulardifliculty has been experienced in providing such metallic surfaces witha ceramic coating that is effective to protect against oxidation,corrosion, and the like at exposure temperatures which extend into therange of from approximately 1700 F. upwards. The basic problems appearto involve both selection of a ceramic coating which affords adequateprotection for all underlying materials at the elevated temperatures anddeveloping proper adhesion of the ceramic coating to the oxidizable orcorrosion-susceptible metallic surface.

This invention utilizes process steps wherein a part having anoxidizable or corrosion susceptible metallic surface is first providedwith an adhering overlay comprised of nickel and chromium and afterwardsprovided with a high-temperature ceramic protective coating in adheringor fused relation to the nickel-chromium overlay. In the preferredpractice of this invention the nickel and chromium overlay is developedby the sequential application of nickel and chromium strikes prior toapplication of the suitable ceramic material to the processed part. Aswill be described in the specification, however, any of severaltechniques may be employed to develop the necessary nickel-chromiumoverlay.

An important object of this invention is to provide an improved processfor applying a high-temperature ceramic protective coating to anoxidizable or corrosion-susceptible surface of a part to thereby developincreased service life for the part when employed in oxidation orcorrosioncausing environments at temperatures in a temperature rangeextending from approximately 1700 F. to an upper limit of approximately2150 F.

Another object of this invention is to provide a process which may beadvantageously practiced in connection with the application ofhigh-temperature ceramic protective materials to metal surfaces of abase product to obtain improved adhesion of such protective materialsentirely throughout a service temperature range which extends from aslow as approximately -1-00 F. to as high as approximately 2150 F.

Another object of this invention is to provide a product which hasoxidation or corrosion-susceptible surface material withhigh-temperature ceramic protective coating that is characterized by acomplete absence of porosity throughout operating temperature rangeswhich extend, at their upper limit, to approximately 2150 F. and thatdevelops improved protection for all materials beneath the ceramiccoating when subjected to oxidizing atmospheres and the like at suchelevated operating temperatures.

A still further object of this invention is to provide a novel processfor applying high-temperature ceramic protective materials tocorrosion-susceptible metal surfaces of a base product whereby theresulting product is characterized by an improved ability to withstandthermal shock resulting from comparatively. rapid thermal cyclingthroughout a range between the temperature limits of approximately F.and 2150 F., without loss of corrosion protection. 1

A still further object of my invention is to provide an' improvedhigh-temperature ceramic coating for protecting metal surfaces and whichis characterized by improved devitrification properties.

Another object of this invention is to provide a ceramiccoated producthaving improved resistance to chemical attack at operating temperaturesin the range of from approximately 1700" F. to approximately 2150" F.

Another object of my invention is to provide a process for applyinghigh-temperature ceramic protective coatings to oxidizable metalsurfaces and which develops an improved wetting action during coatingfiring operations in air atmospheres at temperatures of approximately1700 F. and above.

Another object of this invention is to provide a process method andresulting product which permits the substitution of non-premium steeland the like for costly corrosion-resistant alloy material in partswhich must be subjected to oxidizing of corrosion-causing atmospheres atelevated temperatures extending from approximately 1700 F. and above andwhich develops improved resistance to oxidation and corrosion incomparison thereto.

Another object of this invention is to provide improved high-temperatureceramic protective coatings for base materials to develop increasedresistance to torsion and bending loadings in the applied coating.

Other objects and advantages of this invention will become apparentduring consideration of the following detailed description.

APPLICATION The invention described herein has particular utilityo andSAE 1020, for example), and tool steels (Types SAE 4130 and H-ll, forexample), enameling iron, copper, and exotic materials such asmolybdenum, niobium (columbium), and tungsten. Likewise, this inventionhas application to compressed graphite or graphite-like components whichhave been previously provided with an exterior or surface ferrous plate.In general, my invention has application to metal-like surfaces whichreadily oxidize at temperatures ranging from red heat values upward. Theinvention is also thought to be useful with respect to surfaces of themetal tantalum.

With respect to specific applications, this invention has been utilizedin the fabrication of brazing 'retorts fabricated of non-premiumlow-carbon cold-rolled steel (SAE 1010). Such retorts are utilized inthe aircraft industry, for example, in connection with the brazing ofstainless or precipitation-hardening steel honeycomb panels incontrolled atmospheres at temperatures ranging up to 2000" a! F. Suchretorts are frequently rapidly cycled from the stated 2000 F.temperature to as low as approximately 100 F.

This invention has also been found particularly useful with respect tojigs, fixtures, and other tools which are used in connection with theforming and heat treating of component parts at high temperatures. Inthe absence of a satisfactory protective coating, such tools exhibit acomparatively short useful life.

, The invention described herein has also successfully been utilized inconnection with stove grates fabricated of non-premium, ferrous metals.In addition, heat sinks of copper metal have been successfullyfabricated utilizing my invention. The coating also serves as ahightemperature electrical insulating coating as when applied to acopper base. Other applications for my invention involve use of theherein-described and claimed process in connection with the manufactureof diesel engine manifolds and steam boiler parts manufactured oflow-carbon steel.. Excellent protection against oxidation and chemicalcorrosion has been obtained for steel pipe hanger devices and for thesteel tanks, pipes, and the like which carry water and/or steam atelevated temperatures.

As used in this application the terms oxidation-susceptible surfaces,corrosion-susceptible surfaces, and the like have reference tosurfaceswhich are comprised of the metals identified in the first fullparagraph under this heading Application. Such surfaces readily oxidizeat temperatures of approximately 1500 F. to 1600 F. and upwards whenexposed thereat to environments or atmospheres which contain air(oxygen) and corrosioncausing chemicals such as water (steam), engineexhaust gases, and the like.

FREE-TREATMENT It is recommended that the metal surfaces of thecomponent parts to be processed in accordance with this invention bepre-treated by suitable cleaning and abrading. Surfaces having oils orlubricants present thereon are preferably Cleaned utilizing conventionalsolvent materials, or vapor degreasing techniques, or emulsion cleaningagents. If only very light oils or fingerprints are present on the metalsurface, cleaning may be accomplished using known commercial alkalinecleaners. Afterwards the part should be rinsed and dried.

It is also recommended that the cleaned metal surface then be abradedusing conventional abrasives. Sand blasting may be accomplished bydelivering No. 40 mesh sharp sand by air blast at approximately 90 to120 pounds per square inch air pressure uniformly over the surface ofthe to-be-processed part. As an alternate, fused alumina grit rangingfrom No. 60 mesh to No. 320 mesh may be substituted for sand. It ispreferred that the air blast delivery pressure be kept to a minimum toavoid warpage when abrading thin, light-Weight, or sheet-like basematerials. 7

PROCESS STEPS The properly pre-treated metal surface of the base part isnext subjected to the several process steps of this invention. Ingeneral, such steps relate to: (l) providing an overlay comprisedessentially of nickel and chromium in adhering relation to thepre-treated metal surface, and (2) applying and fusing ahigh-temperature ceramic protective coating over and to thenickel-chromium overlay. For optimum results I prefer that thenickel-chromium overlay be developed by sequentially applying first anickel strike and then a chromium strike to the base part andsubsequently applying and fusing the high-temperature ceramic protectivecoating over and to the chromium strike. However, excellent results areobtained when the nickel-chromium overlay is developed by flame-sprayinga nickel-chromium alloy of powdered form upon the oxidizable orcorrosion-susceptible surface of the base part. Other techniques forapplying I. Nickel-chromium overlay The step of providing anickel-chromium overlay in adhering relation to an oxidation-susceptibleor corrosionsusceptible metallic surface prior to coating with ahighternperature ceramic is novel and is original with this invention.Such step may be accomplished using any one of several conventionalapplication techniques; in some cases the application may involveseveral sub-process steps. In general, the techniques for developing thenickel-chromium overlay on the surface of the to-bcprocessed partinvolves: (l) sequentially electro-depositing separate nickel andchromium strikes in a prescribed sequence, or (2) simultaneouslydepositing the nickel and chromium constituents in a suitable alloymaterial upon the oxidation-susceptible surface in adhering relation asby a flame-spraying technique, or (3) applying one or both of theoverlay basic constituents to the part exterior oxidizable surface usinga combination of separate electro-deposition, flame-spraying, diffusion,or vacuum deposition process steps in prescribed sequence. As previouslysuggested, optimum results and advantages are obtained in my inventionthrough use of electro-deposition techniques as applied to separatenickel and chromium strikes or films. However, excellent results incomparison to known metal part protection methods are obtained when aflame-spray technique of metal application is employed to develop therequired nickel-chromium overlay. Development work carried on to datealso establishes that diffusion techniques, such as have been developedfor depositing chormium on steel parts, can be advantageously employedin connection with this invention. It should be recognized that theoverlay nickel constituent can be applied best alone by eitherelectro-deposition or by flame-spraying a nickel powder on thepretreated part. Conventional nickel application techniques also involvethe application of a nickel oxide or a nickel compound to the base partfollowed by suitable reduction or disassociation steps. Experience seemsto establish that nickel cannot be provided in the necessary overlaythrough use of a gaseous or vapor diffusion process. Also, the resultsobtained with vacuum deposition of chromium are not always the bestobtainable. In any event, when the nickel and chromium constituents ofthe required nickel-chromium overlay are applied to theoxidation-susceptible surface of the to-be-protected part separately, itis important that the nickel be applied prior to application of thechromium.

The following discussion first provides detail information with respectto the preferred steps of sequential application of nickel and chromiumand is followed by a discussion of details relating to diffusiondeposition and flame-spraying techniques.

A. Nickel strike.in order to obtain the optimum advantages of thisinvention it is preferred to first provide the above-describedpre-treated metal surface with a nickel strike. Such nickel strikecomprises an .electro-deposited metallic nickel film ranging inthickness from approximately 0.0001 to 0.0002". A nickel plate, on theother hand, typically varies from 0.001" to 0.00 in thickness. Suchadded thickness is not required in connection with the practice of thisinvention, and in some instances may prove to be a disadvantage withrespect to properties developed in the end product. The prescribednickel strike may be obtained by practice of the details set forthbelow.

The part, if made of steel or a ferrous alloy, is first reverse-etchedin a sulfuric acid-water solution containing 25% i-2% by weight ofsulfuric acid to further assure the removal of oxides from exposedsurface areas. The reverse-etch bath is maintained at approximately F.and a minimum current of 200 amperes per square foot is conductedthrough the part for a period of approximately 5 minutes. In connectionwith this reverse-etching step the steel part should be made an anode.Afterwards the etched surface is rinsed with cold water.

The nickel strike is next obtained using either a nickel sulfatesolution or nickel sulfamate solution. In the case of the nickel sulfatesolution each gallon of solution water contains 22 to 26 ounces ofnickel sulfate, 3 to 4 ounces of ammonium chloride, and 3.3 to 4.5ounces of boric acid. A preferred nickel sulfamate solution plating bathcontains, in each gallon of solution water, 60 ounces of nickelsulfamate, 4 ounces of boric acid, 0.5 ounce of stress reducer, and 10.2ounces of metallic nickel. In connection with either nickel platingsolution, the pre treated metallic surface is suspended in the bath as acathode. To achieve a satisfactory nickel strike using theabove-described nickel sulfate solution, a current density of 30 to 70amperes per square foot is maintained for a corresponding period of timeranging from approximately 6 minutes to approximately 3 minutes. In thecase of the nickel sulfamate plating solution, plating current densityis maintained at from approximately 40 to 80 amperes per square foot fora comparable period of time. In either case, the final thickness of thenickel strike should range from 0.0001 to 0.0002". Afterwards the partshould be rinsed with tap water.

B. Chromium striker-Next, the pre-treated metal surface having a nickelstrike is provided with a superim posed chromium strike. Such chromiumstrike comprises an electro-deposited metallic chromium film ranging inthickness from approximately 0.0001 to 0.0002. A chromium plate, on theother hand, typically varies from 0.001" to 0.004" in thickness. Suchadded thickness is not required in connection with the practice of thisinvention, and in some instances may prove to be a disadvantage withrespect to properties developed in the end product. The prescribedchromium strike may be obtained by practice of the details set forthbelow.

The nickel-striked metal surface is immersed as a cathode in a platingbath which contains 30 to 34 ounces of chromic acid and .30 to .34ounces of sulfuric acid for each gallon of solution water. The followingcurrent densities may be established for the indicated periods of timeto provide a suitable chromium overstrike: 3.0 amperes per square inchmaintained for approximately 6 minutes, 4.0 amperes per square inchmaintained for approximately 4 /2 minutes, or 5.0 amperes per squareinch maintained for approximately 3 minutes. The bath temperature shouldbe maintained at approximately 135 F. -5 F. Afterwards the part issuitably rinsed and preferably force-dried. It is important that thechromium strike completely cover the previously-applied nickel strike.

C. Diffusion depsiti0n.Numerous techniques exist for diffusing chromiumand other metals into .base parts. In general, the depth of diffusionmay range from 1 to mils and will depend upon the length of time theto-beprocessed part is exposed to the gaseous metal at a particularelevated temperature.

In the practice of this invention the following technique has beenemployed to deposit chromium upon a previously-applied nickel strike ornickel film. The pre-treated part, having a nickel strike or surfacefilm thereon, is placed in a sealed retort with a sufficient quantity ofchromium halide compound. The loaded retort is placed in a furnace andheated to a temperature of from 1650 F. to 2000 F. At'the prescribedelevated temperature, the chromium-containing compound is decomposed andchromium metal vapors are released Within the retort. The parts andloaded retort are maintained at the elevated temperature for from 8 to16 hours to permit the diffusion of chromium into the nickel-coated,oxidation-susceptible surface of the metal part. Afterwards, the retortand parts are' cooled and the parts removed for subsequent applicationof the required high-temperature ceramic protective coating asdescribedbelow.

D. Flame-spraying deposition-If a flame-spraying technique fordeveloping the required nickel-chromium overlay on the pre-treated metalpart is prefered in whole or in part to an electro-deposition techniquefor reasons of ease of application, the following should be considered.

An alloy metal in powdered (or wire) form and comprised essentially ofnickel and chromium is preferred for use in connection with aflame-spraying technique, one

satisfactory material which has been employed for this purpose is apowdered alloy containing approximately,

% nickel and approximately 20% chromium. Other nickel-chromium alloyscontaining approximately equal parts of nickel and chromium, andnickel-chromium-iron alloys have generally also proved acceptable foruse with this invention. Such materials are typically fed into aconventional flame-spraying gun having a feed hopper and operating witha combination of oxygen and acetylene gases. The metal powder (or wire)passes through the equipment gun flame having a temperature of from 3000F. to 4000" F., is melted, and is projected by a carrierv gas onto thesurface of the pre-treated and to-be-processed part. Such operationeffects a mechanical bond upon cooling of the melted material and bestresults are obtained if the carrier gas is selected so as to provide aslightly reducing atmosphere at the surface of the part. In this mannerundesirable oxidation or corrosion of the surface may be substantiallyavoided.

Sufficient alloy material is applied to the part to develop anessentially nickel-chromium overlay having a thickness of approximately2 to 3 mils. During the subsequent process step of firing ahigh-temperature ceramic protective coating to the overlay, the ceramiccoating operates to seal off the oxidizable or corrosion-susceptiblesurface and the nickel-chromium overlay from exposure to oxygen and thelike in the firing furnace atmosphere. At the elevated ceramic materialfiring temperature the nickelchromium overlay forms a solid solutionwith the base metal and forms a tightly adherent skin under thehightemperature ceramic protective coating. The subsequently providedhigh-temperature ceramic coating fuses well with the skin; also,improved adhesion of the ceramic to the skin is developed through thedissolving of any oxides formed during coating firing in the glass phaseof the hereinafter-described ceramic coating compositions.

II. Coating application Next, a suitable high-temperature ceramicprotective coating is applied to the metal surface over thenickelchromium overlay. Detailed information with respect to thecomposition of ceramic protective coatings which have provensatisfactory for use in connection with this invention will be providedhereinafter. In general, such materials are particularly selected forspecific applications. Material characteristics with respect toapplication techniques, firing temperature, flow properties, refractoryqualities, expansion-contraction characteristics, adhesion, andhigh-temperature thermal endurance are developed using formulationtechniques which, at least in part, are known to those skilled in theart.

The ceramic protective coating is comprised of both glass frit andrefractory constituents; application may be by either spraying, dipping,or sloshing a slurry mixture (slip) followed by suitable drying. Ingeneral, such ceramic protective materials are provided over thechromium strike to a depth which will result in a final fired coatingthickness of 0.001 to 0.002".

The applied coating, in the case of glass frit-refractory slip mixtures,is afterwards fused to the nickel-chromium strike combination or overlayby furnace firing in an oxygen-containing (air) atmosphere attemperature of from approximately 1700 F. to 2200 F. The firing scheduleactually selected depends upon the composition of-the coating material.Conventional furnace equipment and firing practices are employed tocarry out the coating firing operation.

7 CERAMIC PROTECTIVE COATING MATERIALS A glass frit-refractory type ofprotective coating which is preferred in the practice of this inventionmay be developed through use of a slip having, by weight:

Parts Glass frit 100 Refractory 2-100 Suspension agent /21O Water 40-70Such slip is preferably applied to the metallic base or metallic surfacewhich is to be protected against corrosion and high temperatures byeither brushing, spraying, dipping, or sloshing. The lower limit forparts by Weight of refractory contained in the protective coating andcontained in the slip is established with due consideration to theamount of refractory material, if any, contained in the glass frit.Detailed information will be provided hereinafter with respect to theglass frit, refractory, and suspension agent portions of theabove-indicated protective coating slip. Such information will establisha better understanding, as to the high-temperature glass frit-refractorytype of ceramic coatings which 1 preferably employ in the practice ofthis invention.

1. Glass frit Glass frits for the practice of this invention include, incombination, glass and a refractory additive and may be grouped into twocategories. One category consists of glass frits which are essentiallybased upon the presence of a devitrite-type glass. Such frits aredetailed by the Example A and Example B glass frit compositions providedbelow. The other category of glass frits have been compounded to provideparticular or controlled thermal expansion characteristics and areselected and combined in proper proportions to comprise an improvedcoating slip as hereinafter-described. Detaiis regarding such glassfrits are provided in the Example G, Example H, and Example 1 glass fritcompositions provided in this description. In general, a prepared(calcined) refractory oxide composition or a mixture of refractoryoxides are added to glass-forming constituents at the time the glassfrit batch ingredients are mixed for smelting.

A suitable devitrite glass for use in the glass frit employed in thisinventtion is based on a sodium-calciumsilicate glass system such asthat referenced in the publication, Phase Diagrams for Ceramists, byLevin, Mc- Murdie, and Hall, 1956, American Society, Inc, Columbus,Ohio, at page 168, Figure 460. Preference is generally given to a glasssystem falling within the lower portion of the devitrite area delineatedin such reference. In general, glass systems containing the devitritecomposition Na O.3CaO.6SiO exhibit improved resistance todevitrification at high temperatures over extended periods of time.Additions of boric oxide to the devitrite glass system compositionincrease the effective devitrification area and permit the subsequentincorporation of other refractory oxides into the basic glass to obtainimproved properties relating to thermal endurance, flow, refractorincss,thermal expansion-contraction fit to the base metal, adhesion, color,and/or surface emissivity.

A representative cross-section of refractory additives which may becombined with the basic glass to establish a suitable frit includes theoxides of: nickel, chromium, aluminum, silicon, titanium, zirconium,iron, manganese, molybdenum, cobalt, cerium, niobium, vanadium,beryllium, and tin. Any such oxide, alone or calcined or otherwisecombined with other such oxides, are added to the basic glass to effectthe desired resultant physical properties for the glass frit. Thepercent weight of added refractory depends upon the degree of solubilityof the particular oxide in the basic glass system. For example, theoxides of titanium, iron, manganese, and niobium are comparativelysoluble in the glass during smelting and function to add thermalendurance qualities to the protective coating without developingexcessive refractoriness. The oxides of cerium and cobalt are moderatelysoluble in the basic glass and cannot be added in large quantitieswithout. effecting a loss of flow. The oxides of nickel, chromium,aluminum, zirconium, and beryllium are least soluble in the glass andsmall quantity additions thereof operate to develop refractoriness, heatresistance, and reduced flow characteristics in the resulting glassfrit.

The ingredients whichcomprise the glass frit are Weighed in a batchcomposition, thoroughly mixed, smelted, and then quenched. Two preferreddevitrite glass frits are detailed as to compositions in the followingexamples:

EXAMPLE AGLASS FRIT COMPOSITION [Parts by weight] Ingredient PreferredAmount [Parts by weight] Ingredient Range Preferred Amount The preferreddevitrite glass frit compositions set forth in connection with Example Amay be developed by smelting the following glass frit batchingingredients in the indicated amounts by weight at 2550 F. until finedand afterwards quenching the molten composition:

Parts Silica 51.7 Anhydrous soda ash 14.6 Sodium nitrate 13.7 Fluorspar3.0 Dehydrated borax 5.0 Calcium carbonate 2.8 Titanium dioxide 5.3Example C refractory oxide 1.8

If it is desired to reduce the surface tension characteristic of theresulting glass frit in its molten state, manganese oxide in the amountof 0.5 to 2.5 parts by weight should be included in the glass fritcomposition. 2.5 parts by weight of manganese oxide are obtained in thefrit composition by adding 2.1 parts by weight of manganese dioxide tothe frit batching ingredients identified in this particular paragraph.

The preferred devitrite glass frit composition set forth in connectionwith Example B may be developed by compounding the following glass fn'tbatching ingredients in the indicated amounts by weight:

Parts Silica 39.3 Calcium carbonate 32.8 Anhydrous soda ash 11.7. Fusedborax 16.2

To prepare the frit ingredients for addition to a slip mixture, theglass and refractory oxide components are drymixed, smelted at 250i) F.until fined, and subsequently quenched and ground.

An improved high-temperature ceramic protective coating which isespecially suitable for the protection of the surfaces of steel partsmay be developed using a combination of particularly selected glass frithaving predetermined thermal expansion characteristics. Detailsregarding separate high, medium, and low thermal expansion glass fritsare provided in the following Examples G, H, and J, respectively. As inthe case of the Example B glass frit, the refractory oxide constituentswhich are added to the frit batching ingredients occur as a mixture ofoxides rather than as a prepared and separately calcined refractorycomposition.

The ingredients which are used to comprise the preferred high, medium,and low thermal expansion glass frits are weighed in a batchcomposition, thoroughly mixed, smelted, and then quenched. Such glassfrits are detailed as to composition in the following examples:

EXAMPLE G-GLASS FRIT COMPOSITION [Parts by weight] Ingredient RangePreferred Amount SiOz- 42. 0-54. 0 48. 0 B203 2. 06.0 4.0 A1201 0. -2.0 1. 0 ZnO 6 0-12. 0 9. 0 K 6. 0-8. 0 7. 0 N820 0. 5-2. 0 1. 0 BaO 21.0-39. 0 30.0

EXAMPLE IF-GLASS FRIT COMPOSITION [Parts by Weight] Ingredient RangePreferred Amount SiOz- 38. 0-43. 0 40. 0 B203. 4. 0-8. 0 6. 0 AlzOw 2.0-4. 0 3. 0 ZnO 6. 0-12. 0 9. 0 BaO- 4. 0-50. 0 42. 0

EXAMPLE J GLASS FRIT COMPOSITION [Parts by Weight] Ingredient RangePreferred Amount SiOz 77. 0-84.0 81.0 13203.. i 9. O-17. 0 13. 0 K2O+NaO 2.0-5.0 3. 8 A1201 1. 0-3. 0 2. 2

The preferred high thermal expansion glass frit composition set forth inconnection with Example G may be developed by smelting the followingglass frit batching ingredients in the indicated amounts by weight at2500 F. to 2650 F. until free of bubbles and afterwards quenching themolten composition: a

The preferred medium thermal expansion glass frit composition set forthin connection with Example H may be developed by smelting the followingglass frit batching ingredients in the indicated amounts by weight at2500 F. to 2650 F. until free of bubbles and afterwards quenching themolten composition:

The preferred low thermal expansion glass frit composition set forth inconnection with Example I may be developed by smelting the followingglass frit batching ingredients in the indicated amounts by weight at2500"- F. to 2650 F. until free of bubbles and afterwards quenching themolten composition:

' Parts Silica 752.0 Nepheline-syenite 93.0 Anhydrous borax 80.0 Boricoxide 75.0

II. Refractory;

The refractory material selected and used as a mill addition to theceramic protective coating slip is generally comprised of one or more ofthe refractory oxides identified above in connection with thedescription of the glass frit refractory additive. The refractorymaterial is milladded to the slip to produce the desired firingtemperature, maturing temperature, and coefiicient of thermalexpansion-contraction to fit the base metal or metal surface of theprocessed product. The amount of refractory included in the slipcomposition depends upon the refractoriness of the glass frit used inthe slip. For instance, a comparatively high percentage of refractoryoxide melted into the basic glass increases the frit meltingtemperature, reduces its flow characteristics at the maturingtemperature desired, and would be used with relatively less additionalrefractory, if any, in the slip mill charge. On the other hand, a glassfrit having a low percentage of refractory additive melted into thebasic glass would have a comparatively low melting temperature and wouldhave an increased flow characteristic at. the desired maturingtemperature. A comparatively larger percentage of refractory materialwouldbe combined with such a glass frit to comprise the slipcomposition.

Generally, I prefer that the ceramic protective coating contains aproper total quantity of refractory whereby sufiicient flow is developedduring firing to completely eliminate coating porosity within the firsttwo or three minutes of the firing operation. I 1

One particular refractory composition which may be employed in thepractice of my invention has the ingredients set forth in the followingexample:

The recited constituents of the Example C refractory are preferablyball-milled with an equal quantity by weight of distilled water untilintimately mixed. The milled ingredients are then dried and afterwardscalcined to 2600 F. for 2 hours. The calcined product is then powderedto -200 mesh size. If the Example -C refractory is added to a basicglass to comprise the glass frit it is 1. 1 preferred that the materialbe added to the frit batch during smelting. If necessary, the indicatedrefractory oxides can be added to theparticular base composition in anuncalcined or raw condition. Generally, this practice is not preferred.

Another refractory composition which may be successfully employed as therefractory portion of the glass fritrefractory slip mixture includes theingredients set forth as follows:

EXAMPLE DiREFRACTORY' [Parts by weight] The above-recited constituentsdiffer from the composition of the Example 0 refractory in the additionof the barium silicate additive. In general, the barium silicatematerial is ground to 200 mesh and thoroughly mixed with the Example Cformulation before calcination.

The above-detailed example refractories are each mixed and calcinedprior to addition to either a ceramic coating slip or to a glass fritbatching mixture. However, the following refractory is essentially onlya mixture of refractory oxides and may be employed advantageously in theceramic coating slip formulation which is hereinafterdescribed inconnection with Example L. For identification purposes, this particularrefractory oxide mixture is designated Example K and has the followingcomposition:

EXAMPLE K--REFRACTORY [Parts by weight] III. Suspension agent Theabove-identified glass slip typically includse a suspension agent itomaintain proper dispersion of the glass frit and refractory in either awater or oil vehicle. It is preferred that either enamelers clay orbentonite be used as a suspension agent in connection with thisinvention. Normally, a relatively lesser quantity of bentonite isrequired if such is used in place of enamelers clay. A good grade ofpurified bentonite, as commonly used with porcelain enamel materials isrecommended. If enamelers clay is employed, a water-washed, air-floatedenamelers grade of clay having moderate to high set is preferred.

APPLICATION EXAMPLES The following information relatesto use-of theinvention in connection with the fabrication of panels for a brazingretort. Sheet-like low-carbonsteel panels were formed to size andprovided with the vapor degreasing and abrasive cleaning treatmentspreviously described herein.

methods described under the heading Process Steps.

Subsequent thereto, the processed panels were coated with a ceramic-typematerial having a slip with the following preferred composition:

EXAMPLE E"SLIP [Parts by weight] Ingredients Range Preferred AmountExample A" Glass Frit; 70-100 Enamelers Clay 5-10 7 Bentonito 0-1 0Example D Refractory 2-30 15 Sodium Nitrite -}4 M Water 40-70 55 Theabove-listed slip batching ingredients were milled to a trace on a 325mesh screen (relative to a 100 gram sample). The milled slip wasafterwards applied to the pre-treated low-carbon .steel panels over thecombined nickel and chromium strikes to a sufiicient depth whereby theresulting coating, after firing to maturity at 1800 F. to 1900 F.,developed a thickness of from .001" to .002" After complete manufacture,the ceramic coated retort panels processed in accordance with thisinvention were thermally cycled in a comparatively rapid manner over therange of 1750 F. to 70 F. in an air atmosphere for 19 complete cycles.At the end of such cycling no failure or deterioration of the resultingnon-porous coating could be observed and there was no oxidation orcorrosion of the metar contained in the retort panels.

The invention described in this application has also been used inconnection with copper bar stock. A copper bar workpiece was firstprovided with the pre-treatment cleaning and abrading operationsdescribed herein. Afterwards the bar was provided with a nickel striketo a depth of 0.00015 using the method described herein; immediatelythereafter a chromium strike was superimposed upon the nickel strike toa like depth using the method set forth under the heading Process Steps.

The so-processed copper bar was then coated with a ceramic protectivecoating slip having the following mill composition:

EXAMPLE F"PROTECTIVE COATING SLIP of the slip ingredients identified inconnection with Example E. Application of the slip to the copper partcorresponded to the application technique described in connectionwiththe low-carbon steel brazing retort panels except that the slipmaterials were fired to maturity at a temperature at from 1800 F. to1850 F.

The so-coated copper bar was subjected to 4 repeated cycles wherein thepart was heated to a temperature of 1850 F. for one-half hour andafterwards quenched in tap water at room temperature. The improvednonporous protective coating exhibited no failure with respect to eitherporosity or adhesion. In addition, the copper part exhibited nocorrosion or oxidation due to the air atmosphere in which it was heated.v

I T he invention described herein has also been employed in connectionwith the coating of common steel boiler plate stock having thicknessesof approximately /s" and /4Q The to-be-protectedboiler plate parts werelater used as a heat barrier in a rocket launching pad. Thehigh-temperature ceramic protective coating described below attainedexcellent protection of the base metal against oxidation and corrosionat elevated temperatures.

The boiler plate parts were first provided with the pretreatmentcleaning and abrading operations described herein. Afterwards, the partswere provided with a nickel- Ingredients Range Preferred Amount ExampleG Glass Frit 35. 040. 37. Example H Glass Frit 10. 015. 0 12. 5 Example3 Glass Frit 35. 0-40. 0 37. 5 Example K Refractory 12.0-18.0 15. 0Annealed Nickel Powder (-325 Mesh). 7. O-8. 0 7. 5 Euemelers Clay 4.0-7. 0 5.0 Distilled Water 50. 0 0.0 60.0

The slip ingredients listed above were milled in the manner of the slipingredients identified in connection with Examples E and F. Applicationof the coating slip to the boiler plate parts was by a sprayingtechnique and included the covering of previously-provided weld joints.The applied slip, after suitable drying, was fired to maturity at atemperature of 1800 F. to 1850 F.

The coated parts were subjected to ten repeated cycles wherein each partwas heated to a temperature of 1700 F. to 1750 F. and immediatelyquenched in tap Water at room temperature. In addition, the parts werecontinuously maintained in an air atmosphere at a temperature of atleast 1700 F. for over 500 hours. There was no detectable failure of thehigh-temperature ceramic protective coating and no corrosion oroxidation of the base metal protected thereby.

The high-temperature ceramic protective coating described in connectionwith the composition of Example L appear to offer advantages over thepreviously-detailed ceramic coating slips having devitrite-type glass inthat added resistance to chemical attack or corrosion at elevatedtemperatures is developed. Also, the high-temperature ceramic protectivecoating of Example L appears to have a greater versatility for effectingthermal expansion fits to base metals having a relatively wide range ofthermal expansion characteristics in comparison to the limited expansionrange capability afforded by the devitrite-type glass materials.However, both coating formulations provided a protective coating whichdid not fail after repeated extreme thermal shock and prolonged exposureto high-temperature environment conditions.

Further, the invention described herein has been utilized in connectionwith workpieces fabricated of molybdenum. Molybdenum panels formed of0.020" sheet stock were first cleaned and abraded using the techniquesheretofore-described. Afterwards, such panels were provided with anickel film having a thickness of from 0.0001 to 0.0002" using thenickel electro-depositing steps separately described above. The nickelcoated panels were then provided with an electro-deposited chromium filmhaving a thickness of from 0.0001" to 0.0002" using the chromium striketreatment described under the heading Process Steps. The molybdenumpanels having the nickel-chromium combination overlay were next coatedwith a ceramic protective coating slip having a mill composition whichcorresponded to the preferred composition set forth in connection withExample E except that 25 parts by weight of Example D refractory wereused instead of 15 parts thereof. The slip material was applied to themolybdenum panels and fired to maturity at an approximate temperature of2000 F. The ceramic-coated panels were afterwards repeatedly cycledbetween the temperature extremes of 1950 F. and F. in an air atmosphere.After performing numerous thermal cycles the molybdenum workpieces wereexamined for coating failure and/ or metal oxidation but no suchdeficiency existed in connection with the coated part.

The invention described herein has also been employed in connection withalloy materials (e.g., ultra-highstrength steels) which receive a heattreatment at elevated temperatures to develop improved strength therein.Such workpieces are typically heat treated in an air atmosphere and areoften subjected to metal decarburization and the like. My invention hasproved effective to protect the alloy material during heat treatment andin some instances need not be removed from the workpiece subsequent tothe heat treatment operation. Coating removal is not necessary becauseof the superior bending and torsion resistance that is developed in theceramic material.

I claim:

In a coated workpiece, in combination:

(a) A surface which constitutes a base interface of oxidizable metalselected from the group consisting of iron, alloys of iron, copper,molybdenum, niobium, and tungsten,

(b) A complex metal consisting of nickel, chromium, and said oxidizablemetal adhered to the workpiece at said base interface, and

(c) A non-porous high-temperature ceramic coating consisting of glassfrit, refractory oxide, and oxides formed and in solution with saidcomplex metal adhered to said complex metal to comprise the exteriorsurface of the coated workpiece,

said glass frit and said refractory oxide being combined in the ratio ofapproximately parts by weight of glass frit to from 2 to 100 parts byweight of refractory oxide, said glass frit being from the groupconsisting of devitrite-type glass frits and of substantiallyfluorine-free glass frits having calculated alkali metal oxide contentsof less than approximately 10% on a weight basis and smeltingtemperatures in the range of approximately 2500 F. to 2650 F., and saidrefractory oxide being from the group consisting of the oxides ofaluminum, beryllium, cerium, cobalt, chromium, iron, manganese,molybdenum, nickel, niobium, silicon, tin, titanium, and zirconium.

References Qited by the Examiner UNITED STATES PATENTS 2,696,662 12/54Le Sech.

2,716,271 8/55 Higgins 1l7-70 2,781,636 2/57 Brandes 11770 3,006,782 10/61 Weildon.

3,069,760 12/62 Schultz.

3,086,284 4/63 Schetky 29-l95 HY LAND BIZOT, Primary Examiner.

JOHN C. MARTIN, IR., DAVID L. RECK, Examiners.

